The number of websites showing illegal images of child abuse has more than doubled, according to police figures .

In its annual assessment of the threat posed in the UK by serious and organised crime, the National Criminal Intelligence Service also says online child sex offenders are becoming much more cunning in their attempts to avoid detection.

The Internet Watch Foundation, which alerts the police to websites containing images of child abuse, saw a 64% rise in the number of reports of illegal sites it received in 2002 compared with 2003.

Most are based in the United States - but the number in Russia also more than doubled.

And the sites that are reported are just the tip of the iceberg, according to police.

Illegal images

Child porn offences on the net carry tough custodial sentences.

To avoid detection, members of online child sex networks attack with viruses the computers of those who have put others at risk by the unauthorised passing on of illegal images.

Paedophiles are naturally suspicious of newcomers into their social circle and, like many criminal groups operating in cyberspace, are skilled at counter-surveillance.

They also debrief people who have been arrested and try to monitor the methods employed by law enforcers.